1. Field of the Invention
This invention relates to a water-based stain for staining wood containing at least one dye, such as azo, diazo and triarylmethane dyes, and to a method for staining wood. In particular, the present invention relates to use of a stabilizing agent which may be a buffer and/or a complexing agent and which is effective to reduce the concentration of free iron ions in the stain from contact with an autoclave composed of non-rust-resistant steel, and thereby reduce coloration impairment of the stain.
2. Background of the Related Art
Wood stains are known, for example, from the furniture industry and are used for staining the surface of a piece of furniture while preserving its natural grain if possible. For this purpose, the surfaces of the pieces of furniture are treated with the stain by, for example, painting or spraying, and any other method know in the art. In such treatment processes the stain penetrates the wood surface only slightly. However, this is often not sufficient for achieving the desired staining of the wood surface. There are also cases where not only a surface staining of the wood is desired, but staining that penetrates deeper into or even entirely through the wood. The latter is the case in connection with the manufacture of lead pencils and colored pencils wherein the body of wood enclosing the lead of the pencil is intended to have a continuously uniform coloration. A further example of applications where penetrating staining of wood is desired is for wood blanks which are intended to be worked later, for example, by cutting or sawing, without the plain, unstained wood appearing at the worked places.
Such staining processes are usually performed in autoclaves. For this purpose, small pieces of wood, e.g., wood blanks in the form of small boards in the case of lead pencil or colored pencil manufacture, are placed into an autoclave and dipped into a colored stain. Then the autoclave is closed and the staining or penetrating staining is started by means of temperature and pressure increases as is well known in the art. Depending on the type of wood and the thickness of the wood blanks to be stained, penetrating staining can take several hours or even several days.
The dyes used can naturally be from a very broad spectrum of different dye types. However, as a rule two groups of dyes are used for staining wood, namely, acid dyes, which are also called anionic dyes, and basic or cationic dyes.
While no problems worth mentioning regarding the stability of the stains occur in connection with any of the mentioned dye groups in autoclaves of rust-resistant steel, for example, V2A steel, it was noted that the dyes were increasingly impaired during continued treatment when acid dyes were used in autoclaves made of normal, i.e., not anti-corrosive steel. The impairment consists in an increasing change in the color of the added dye.
The color change effect occurs in particular with azo- and diazo-dyes, but in a weakened form, also with triarylmethane dyes. It is known that acid dyes are used in a neutral to alkaline environment. At the end of the dying process the pH value of the stain typically has been lowered to a varying degree into the acid range, depending on the type of wood used. The reason for this is that organic acids are contained in the wood which cause an increase in the H.sub.3 O+ ion concentration. However, this effect cannot be or at least cannot be totally responsible for the observed color changes, since generally no color changes were observed in connection with autoclaves of special steel in spite of the lowering of the pH values.
Based on the above mentioned observation, it is the object of the present invention to reduce or eliminate the prior art color change effect by providing a stain and a staining method for wood which can be used in autoclaves made of non-rust-resistant steel and while using azo, diazo and triarylmethane dyes.